1. Field of the Invention
This invention relates to a detector for measuring the electrical conductivity of planar panels such as sheets of glass having a coated surface on one side thereon, and the detector is particularly adapted for convenient, single handed use in order to free the other hand of the worker for other tasks.
2. Description of the Prior Art
Glazing having a low emissivity coating has been used in an ever-growing share of energy efficient residential and commercial construction in the past few years. Low emissivity glass, or "low-E" glass, includes a thin layer of a coating, typically metal oxide, that is applied to one side of the glass and functions as a selective filter that allows light and near-infrared radiation to pass through the glazing and into the building, while also reflecting long-wave heat that is radiated from materials within the building back toward the interior of the latter. Low emissivity coatings on glass are known to substantially increase the thermal resistance, or R-value, of the glazing, and thereby correspondingly reduce the energy costs for heating and/or cooling of the building. Low emissivity coated glass also offers other advantages over clear, uncoated glass, such as greater comfort to the occupants of the building and reduced fading of fabrics.
Glazing manufacturers increase the thermal resistance of their products by adding additional panes to form dead air spaces which reduce thermal conduction losses. However, a double-glazed window can transfer up to approximately twice as much heat by thermal radiation as compared to the amount of heat transferred by conduction, and thus it is particularly desirable to use a low emissivity coating on one surface of window units which are formed of two spaced, glass panes. In double-glazed units, the low emissivity coating is known to be beneficially effective only when applied to an inner surface of one of the panes in facing relationship to the dead air space.
Currently, two processes are in popular use for coating glass or films. Relatively soft low emissivity coatings are applied by a vacuum deposition process wherein a metal bar is bombarded with electrons and atoms emitted from the bar condense in a vacuum on a cold surface of the glass. Relatively hard low emissivity coatings are made by a pyrolitic process by spraying a hot, thin, metallic oxide coating onto hot glass such that the coating bakes onto the glass surface and hardens as the glass cools. Low emissivity coatings can be comprised of a number of different materials, such as tin oxides or other metallic oxides, or, in some instances, silver or copper.
As mentioned previously, it is of utmost importance that the side of the glass panel which has the low emissivity coating be positioned to face the interior of the dead air insulative space in insulated units which include two or more panes of glass in order for the coating to effectively increase the thermal resistance of the unit. If the glass surface having the coating is mistakenly positioned on an outside surface of one pane of a unit, the problem can go undetected for years and energy costs for the conditioned space will be high. Moreover, the glass of double-glazed units cannot be easily reversed, since such units are permanently sealed at the factory and include a desiccant in the air space to reduce condensation between the panes.
However, during factory assembly of double-paned units, it is generally impossible to visually determine which side, if any, of a glass panel carries a low emissivity coating. In the past, factory workers have typically used a conventional volt-ohmmeter having two flexible, wire-like probes which are both moved by the worker to contact a single side of the glass sheet, since the side having the low emissivity coating exhibits a substantially higher electrical conductivity than the side without the coating. As can be appreciated, use of the ohmmeter with the wire-like probes is somewhat difficult, since it is necessary to use both hands for maneuvering the meter and probes, and consequently the sheet of glass cannot be easily supported by the user at the same time.